The vapour pressure of pure liquid solvent A is 0.80atm.When a non volatile substance B is added to the solvent,its vapour pressure drops to 0.40atm,the mole fraction of component B in the solution is

Question

$\begin{array}{1 1}(a)\;0.25\\(b)\;0.75\\(c)\;0.50\\(d)\;0.40\end{array}$